Since a non-contact type IC card and an RFID tag having a coil therein as well as a sensor for a reader/writer used together with the IC card and the RFID tag are actuated in a magnetic field or an electric field generated by a high frequency vibration, their sensitivities are greatly deteriorated due to a mirror effect which offsets the magnetic field or the electric field, if such sensors are placed closely to a metal bed or metal surface.
This is due to a phenomenon that the electric field or a magnetic field around the sensor is compensated with a generated electric field or a magnetic field by a reverse phased current due to the mirror effect. In other words, properties of such sensor are spoiled, compared with a case when the metal bed is not applied closely to the sensor.
There is a structure called “on metal”, where a magnetic sheet is inserted between a coil of the RFID and the metal surface in order to prevent the sensitivities from being deteriorated. Although this structure is effective to a certain extent, merely a portion of magnetic field directs to a vertical direction to the metal surface, since the magnetic field is influenced by the metal surface and since magnetic field components parallel to and near the metal surface are compensated each other.
Since only a center portion of a circularly wound coil generates vertical components of the magnetic field, it is difficult to obtain a strong magnetic field. Therefore this method is considered to be rather a passive one.
In a method proposed and filed as a patent application (not disclosed yet) by the applicant, a magnetic flux density is doubled, namely a voltage is increased by 6 dB by utilizing the mirror effect. This method can intensify a magnetic field mainly in a direction along the metal face as well as a vertical direction, but vertical magnetic field components are attained only in limited areas close to both magnetic poles.
In another method proposed and filed as another patent application (not disclosed yet) by the applicant, strong vertical magnetic field components are obtained in the center of the magnetic field by utilizing combined rods of a square cross-section. However, sometimes it is difficult to obtain such combination of the rods, when a hole is required on the surface of the combined rods for burying the tag.